If avoiding poop is the goal then I'd recommend whomever is bottoming clean themselves out by douching first. I've been an exclusive bottom since I started having sex, and I can tell you that I always would take a good 20 minutes before sex to make sure I was squeaky clean. My ex would good annoyed by it, because he didn't like that we couldn't be spontaneous about things, and that if we got frisky I'd be like "I'll be back in about 15 minutes." However, that's me. I was paranoid by the idea that I wouldn't be spotless and something would come out, so that's the standard I place on myself. That being said even with the precautions there were a couple occasions when there was a small speck of something that ended up on his dick. Of course in those circumstances I'd end up being mortified, but "c'est la vie."
One thing that I can say is that douching, aside from making you as clean as possible, will also greatly reduce any of the unpleasant aromas that can occur with anal sex. That's another reason I always wanted to make sure I took time to prepare things before doing the deed. Hahaha.
